Polypropylene resins are used in various fields due to the excellent characteristics and inexpensiveness. Propylene homopolymers generally have a high rigidity, but are inferior in impact resistance, particularly at low temperatures.
A propylene block copolymer having improved low temperature impact resistance of a homopolymer is known. The copolymer is prepared by incorporating a first formed component propylene homopolymer segment and a second formed component ethylene/propylene random copolymer segment. This block copolymer is widely used in industrial fields including automobiles and home appliances.
Such block copolymers are excellent in impact resistance, but are inferior in transparency and gloss as compared with a homopolymer. Further, the copolymers have a high mold shrinkage factor as is the case with the homopolymer. In order to improve this, inorganic fillers such as talc are used or an ethylene content in the random copolymer segment is reduced. The former, however, brings about a weight increase and poor appearance of the molded article due to the addition of a large amount of inorganic substance. The latter reduces the rigidity as well as the impact resistance at low temperatures, though the transparency and gloss are elevated.
With these problems, JP-B-7-30145 discloses a propylene block copolymer comprising a crystalline polypropylene segment and an ethylene/propylene random copolymer block segment, wherein the crystalline polypropylene content is 55-95% by weight; an intrinsic viscosity ratio of both the segments [.eta.].sub.EP /[.eta.].sub.PP is 0.5-2.0; and the ethylene/propylene random copolymer block segment has a glass transition temperature of -30.degree. C. or lower. Further, JP-A-6-93061 discloses a propylene block copolymer obtained by molten-kneading a block copolymer which is prepared by polymerizing firstly 60-80% by weight, based on the whole polymerized amount, of a polymer mainly composed of propylene, and then an ethylene/propylene copolymer segment having an ethylene content of 20-50% by weight. In the block copolymer, the ethylene/propylene copolymer segment has an intrinsic viscosity [.eta.].sub.B of 2.0 dl/g or more, and an intrinsic viscosity ratio of both segments [.eta.].sub.B /[.eta.].sub.A is 1.8 or less.
However, such propylene block copolymers have the ethylene/propylene copolymer segment contents and an intrinsic viscosity ratio which are controlled almost in the same ranges. The former case has improved mechanical characteristics such as impact resistance and rigidity, but is still unsatisfactory in the transparency, gloss and appearance. On the other hand, the latter has improved impact resistance at low temperatures, blocking resistance and appearance, but is still unsatisfactory in the rigidity. Molding shrinkage factor is also not satisfactory in both cases.
The present inventors have proposed propylene block copolymers which are excellent in and well balanced of transparency, gloss, molding shrinkage factor and rigidity, in JP-A-8-27238, WO97/19135 and Japanese Patent Application No. 132176/1997.
On the other hand, soft or semi-rigid polyvinyl chloride has widely been used as exterior or interior materials for automobiles and home appliances due to the easiness in processability. However, they may produce dioxin upon incineration, and thus recently, various alternates have been investigated. Typically used are olefin resins such as olefin thermoplastic elastomers.
While the propylene block copolymers proposed by the present inventors have excellent and well balanced characteristics as mentioned above, they are not fully satisfactory particularly when used for exterior or interior materials and are therefore still a matter for improvement.
On the other hand, the thermoplastic elastomers described above are not only costly, but also have poor flowability on molding, so that the resulting molded articles (exterior and interior parts) often have an unsatisfactory appearance. In particular, they are inferior in the emboss transferring property from a mold and an embossing roll and are apt to damage the high quality impression of the final home appliances and automobiles.
The present invention has been made based on these problems in the conventional techniques, aiming to provide a polypropylene composition which is of low cost and excellent in the emboss transferring property and useful for various exterior and interior materials.
In order to achieve the above object, the present inventors have intensively studied and found that blending a specific propylene block copolymer component with a rubber component and controlling the melt flow rate (hereinafter abbreviated to "MFR"), if necessary, can achieve the object.